Citric Acid is produced worldwide by fermentation of carbohydrates followed by multi-step purification and recovery processes. Although many micro-organisms are known to produce citric acid, only two--Aspergillus niger a fungus, and Candida sp. a yeast, have been used for commercial production. Of the two--A. niger strains have been the organisms of choice. Many fermentation carbon sources have been used such as molasses; glucose syrups derived from hydrolyzed starch of corn, cassava, sweet potatoes and the like; glucose syrups derived from hydrolyzed cellulosic feedstocks, and also hydrocarbons and the like as are well known.
Under normal fermentation conditions, the A. niger strains do not produce citric acid in high yields or in high concentrations, often because of feed stock contamination. When molasses is used as the carbon source for example, many undesirable constituents have to be removed prior to use. Methods of molasses purification include use of ferrocyanide salts precipitation, ion-exchange, and carbon adsorption. When glucose syrup from starch sources are used, the purification steps to remove the undesirable constituents are less onerous but ion-exchange and/or adsorption are required.
Fermentation by A. niger in submerged culture also requires the development of the suitable inoculum. In many cases this involves inoculum propagation in multi-stage fermentors and very careful control of the concentration of potential pellet-forming mycelia in each of the propagation stages. In the production fermentation, mycelial pellets of approximately one millimeter (mm) diameter are formed and these pellets can produce high levels of citric acid with high yields.
If the environmental conditions in the fermentor are not carefully controlled, the acid production can be very adversely reduced. The key environmental control factors are: trace metal concentrations particularly iron, copper, zinc and manganese--maintenance of low but controlled levels (in parts per billion) of manganese is critical; pH value, nitrogen and phosphate levels. Often, control of the above factors and development of specific strains still fail to produce citric acid without contaminating organic acid impurities such as iso-citric, gluconic or oxalic acids. A good recent summary of citric acid production microorganisms is provided in Bigelis et al., Chapter 6 in Food Biotechnology Microorqanisms, Hui and Khachaturians eds., VCH Publishers, New York (1995) pp. 239 ff.
The conventional citric acid recovery and purification process is a unwieldy multi-step process that generates waste gypsum. That process usually involves ten steps, several of which are cumbersome solid-liquid separations.
Thus, the A. niger mycelia are separated from the fermentation broth by settling followed by rotary vacuum filtration where solid filter-aid is added to form a mycelia filter cake that can be disposed as animal feed. The filtered broth is then neutralized with high grade lime (0.5 lb/lb citric acid), and initially, the oxalate (if any is formed in the broth), is precipitated as the calcium salt and removed by filtration. The lime-containing slurry is then heated first to 80.degree.-90.degree. C. and then to 95.degree. C., and the insoluble calcium citrate is recovered using a rotary filter.
The recovered calcium citrate solid is formed into an aqueous slurry and is then mixed with 95 percent sulfuric acid (0.8 lb/lb citric acid). The now soluble citric acid is removed from the calcium sulfate waste product by another rotary filtration. The citric acid in the sulfuric acid phase is present at a concentration of about 150-200 g/L, and that phase is concentrated in a multiple effect evaporator to approximately 67 weight percent, followed by treatment by ion exchange and/or activated carbon, further evaporation, and the desired citric acid is crystallized, washed and dried. A general description of such a conventional citric acid recovery and purification process is provided in Atkinson et al., Chapter 19 in Biochemical Engineering and biotechnology Handbook, 2nd Ed., Stockton Press, New York (1991) pp. 1098-1100.
The above process creates approximately two pounds of waste gypsum per pound of citric acid recovered. The gypsum cake wash also creates solid wastes in the wash waters. Furthermore, many large and unwieldy solid liquid filters are used and a large amount of energy is required in this multi-stage process that undergoes four phase changes.
For these reasons, many attempts have been made to eliminate the waste gypsum-based recovery and purification process. Liquid-liquid extraction using amine-containing ion exchangers has been used as one of the purification steps. Chromatography has been used to separate the carbohydrates from the organic acids. Although these techniques have met with some successes, they have not been able to produce efficient and relatively waste-free processes. Liquid ion exchange produces solvent-contaminated raffinates that have to be disposed of. The chromatographic separations produce more dilute streams than the feed, and hence the evaporation energy and costs increase. Often, the other organic acids produced by the particular fermentation interfere with the purification and do not produce the high purity citric acid desired.
Electrodialysis is a membrane based process in which ions are transported from one solution into another by application of an electrical driving force. Electrodialysis using desalting membranes has been used to recover salts of organic acids such as lactic, succinic and acetic from fermentation broths. See, for example, U.S. Pat. No. 5,143,834 whose disclosure is incorporated by reference and European Patent Application No. 90301838.1. In those processes however, fermentation produced the salts rather than the acids. The salts had to be further converted to the acids by use of electrodialysis with bipolar membranes, thereby requiring a two step process.
Furthermore any by-product acids produced could not be separated from the product acid by those processes. See, European Patent Application No. 90301838.1. Contrarily, fermentation produces citric acid in the broth, and that product has to be recovered and purified. Any by-product acids, if produced, lead to difficulties in purification because their properties and separation methods are substantially the same as that for the main product.
An improved process should be able to produce citric acid and/or its monovalent salts with high purity, without creating gypsum waste, without multiple phase changes, without organic solvents, require low energy and be of low overall cost. One such process has been discovered as is discussed below. That process involves the integration of an improved method of fermentation with electrodialysis (ED) as the primary means of recovery and purification.